Warm the water: Warm the 375 g of water before you start. For instant yeast, use the temperature on your yeast packet, since brands vary from about 100°F to 120°F. For active dry yeast, warm it to 100°F to 110°F, which should feel warm but not hot on the inside of your wrist
If using instant yeast: In a large bowl or dough container, whisk together the bread flour, instant yeast, salt, and sugar. Instant yeast does not need to be bloomed, so it goes straight into the dry ingredients. Add the warm water, vanilla, melted butter, and cinnamon and mix until no dry flour remains.
If using active dry yeast: Pour the warm water into a large bowl or dough container, stir in the active dry yeast and a pinch of the sugar, and let it sit for about 10 minutes, until the top is foamy. If it does not foam at all, your yeast may be expired, so start again with fresh yeast. Once it is foamy, add the bread flour, salt, remaining sugar, vanilla, melted butter, and cinnamon and mix until no dry flour remains.
Rest: Cover and rest for 30 minutes.
Stretch and fold: With wet hands, pull an edge of the dough up and fold it into the middle four times, working around the bowl. Cover and rest 30 minutes. Repeat for three sets total, resting 30 minutes after each. If the dough still feels slack after the third set, give it a fourth.
Bulk rise: Leave the covered dough at room temperature for 1 to 1½ hours after the final set, until doubled.
Prepare the pan: Line a 9x13 inch metal pan with parchment.
Set up the dip and coating: Stir the melted dipping butter and vanilla together in one small bowl. Mix the brown sugar, cornstarch, and cinnamon together in a second small bowl.
Turn out the dough: Turn the dough out onto a lightly floured counter.
Shape with the Cinnamon Tuck: Pinch off a piece of dough about 79 g. Dip it in the vanilla butter, then roll it generously in the cinnamon sugar. Push your thumbs into the bottom of the piece so the cinnamon sugar is pressed up inside, bring the sides up and seal, then do the same for the opposite edges. Pinch them together underneath into a taut ball. Set it seam side down in the pan, then shape the next piece. Arrange the 12 rolls in three rows of four, close together.
Use the leftover coating: Drizzle any butter between the gaps by the cinnamon rolls. Sprinkle any cinnamon sugar left in the bowl over the tops and into the gaps between the rolls.
Second proof: Cover the pan and let the rolls rise at room temperature for 1 to 2 hours, until they are puffy, soft, and pressed up against each other. Allow longer in a cool kitchen.
Preheat: About 20 minutes before the rolls are ready, preheat the oven to 400°F (200°C) with a rack in the center.
Dimple: Gently press your fingertips into the top of each roll.
Bake: Bake for 25 to 30 minutes, until deep golden and the center roll reads 190°F to 200°F on an instant read thermometer. If your oven runs hot, start checking at 20 minutes, and tent loosely with foil if the tops brown too quickly.
Cool: Let the rolls cool in the pan for 20 to 30 minutes.
Make the icing: Stir the powdered sugar, vanilla, salt, and 15 g of milk together, adding the rest of the milk only if needed, until the icing holds a soft peak but still spreads slowly.
Ice: Scoop a round of icing onto the center of each roll and let it settle. Pull apart and serve warm.